This is a great suggestion. The problem is, S1 isn't calling the standard Windows file save dialog, which always shows files of the type you're saving in the destination folder. S1 is using is own, internal file save dialog. They need to either implement the same functionality in their dialog, or just call the Windows dialog. I have, more than one time, had a mixdown named 'songname(2).wav' because I forgot I had already mixed that song down before and it was named 'songname.wav'.
So, for your purposes, the incrementing behavior is already there, BUT, if you happen to not remember the file name, it won't do that incrementing. So yeah, great suggestion.